Process for the (co)polymerization of ethylene

ABSTRACT

A process for the (co)polymerization ethylene, optionally in mixtures with olefins CH═CHR in which R is hydrogen or a hydrocarbyl radical with 1–12 carbon atoms, carried out in the presence of a catalyst system comprising (A) a solid catalyst component which comprises Mg, halogen an electron donor selected form ethers, esters, or amines, and Ti atoms in an oxidation state such that the weight percentage ratio between Ti (red) /Ti (tot)  ranges from about 0.05 to about 1; wherein Ti (red)  is the weight percentage on the solid catalyst component of the Ti atoms having a valence less than 4 and Ti (tot)  is the weight percentage on the solid catalyst component of all the Ti atoms and (B) an Al-alkyl compound. The said process is capable to produce ethylene polymers with a reduced oligomers content and/or improved mechanical characteristics.

This application is the U.S. national phase of International ApplicationPCT/EP02/02803, filed Mar. 11, 2002.

The present invention relates to a process for the (co)polymerizationethylene, optionally in mixtures with olefins CH₂═CHR in which R ishydrogen or a hydrocarbyl radical with 1–12 carbon atoms, carried out inthe presence of a specific Ziegler-Natta comprising Mg, halogen, anelectron donor compound and Ti atoms in a different valence state. Thesaid process, is capable to give ethylene copolymers with a reducedoligomers content and/or improved mechanical characteristics. In thepolyolefin field Ziegler-Natta supported catalysts are customarily usedfor the preparation of homo or copolymers of olefins such as ethylene,propylene, butene-1 and so forth. These catalysts, and in particularthose using Mg compounds as supports, allow to obtain good products inhigh yields and are sufficiently versatile to be used in several kindsof polymerization processes. Among the various homo and copolymersobtainable with the supported Z/N catalysts, copolymers of ethylene withone or more alpha-olefins having a molar content of units derived fromethylene of higher than 80%, and in particular Linear low-densitypolyethylene, (LLDPE) are the most important products. Due to theircharacteristics, they find application in many sectors and in particularin the field of wrapping and packaging of goods where, for example, theuse of stretchable films based on LLDPE constitutes an application ofsignificant commercial importance. In order to be used in the packagingfield, one of the important requirements for the LLDPE films is the tearresistance which is measured by the Elmendorf test. Usually, thechemical and physical properties of the polymers are adjusted so as togive films showing a threshold acceptable value of Elmendorf. The commonway to reach the requested Elmendorf value is that of lowering thedensity of the LLDPE by introducing a higher amount of alpha-olefincomonomer. However, as the Z/N supported catalysts have the tendency togive a broad compositional distribution, the result is that very often atoo high fraction of highly modified soluble copolymers is obtained.This fraction is responsible for problems both in the carrying out ofthe polymerization process and in the properties of the final productsbecause the low molecular weight soluble polymers have the tendency tomigrate to the surface of the films (blooming) thereby making the filmitself sticky. EP 739907 discloses a solid catalyst component comprisingmagnesium, titanium, halogen and an electron donor compound in which thedivalent titanium atoms account for not more than 25 atomic % of thewhole titanium atom content and trivalent titanium atoms account for atleast 30 atomic % of the whole titanium atom content. According to thesaid document the catalyst obtained by this catalyst component iscapable to form propylene polymers with high stereoregularity and goodyields. In EP 739907 the catalyst system is never used for(co)polymerizing ethylene. It has now been found a novel polymerizationprocess that is able to produce ethylene copolymers having, for acertain density, an increased Elmendorf value. This makes it possible toproduce LLDPE at a requested Elmendorf, with a higher density and aconsequently lower amount of soluble fraction. The process for the(co)polymerization of ethylene, optionally in mixtures with olefinsCH₂═CHR in which R is hydrogen or a hydrocarbon radical with 1–12 carbonatoms, is carried out in the presence of a catalyst system comprising(A) a solid catalyst component which comprises Mg, halogen, an electrondonor selected from ethers, esters, or amines, and Ti atoms in anoxidation state such that the weight percentage ratio betweenTi^((red))/Ti_((tot)) ranges from about 0.05 to about 1; whereinTi^((red)) is the weight percentage on the solid catalyst component ofthe Ti atoms having a valence less than 4 and Ti_((tot)) is the weightpercentage on the solid catalyst component of all the Ti atoms, and (B)an Al-alkyl compound. In particular the ratio Ti^((red))/Ti_((tot))preferably ranges from 0.1 to 0.9, more preferably from 0.2 to 0.85 andstill more preferably from 0.4 to 0.8. It is especially preferred thatthe catalyst components possess the above mentioned features beforebeing subject to conditions such that (pre)polymerization takes place.The electron donor compound is preferably selected from ethers andesters of organic mono or dicarboxylic acids. Among ethers, particularlypreferred are the 1,3-diethers of the general formula (I):

wherein R^(I), R^(II), R^(III), R^(IV), R^(V) and R^(VI) equal ordifferent to each other, are hydrogen or hydrocarbon radicals havingfrom 1 to 18 carbon atoms, and R^(VII) and R^(VIII), equal or differentfrom each other, have the same meaning of R^(I)–R^(VI) except that theycannot be hydrogen; one or more of the R^(I)–R^(VIII) groups can belinked to form a cycle. Particularly preferred are the 1,3-diethers inwhich R^(VII) and R^(VIII) are selected from C₁–C₄ alkyl radicals.

As explained preferred esters are those of organic monocarboxylic ordicarboxylic acids. Said acids can be aliphatic or aromatic. Amongesters of aliphatic acids, particularly preferred are the esters ofdicarboxylic acids in particular esters of malonic, glutaric or succinicacids. Preferred esters of aromatic carboxylic acids are selected fromC_(1–C) ₂₀ alkyl or aryl esters of benzoic and phthalic acids, possiblysubstituted. The alkyl esters of the said acids are preferred.Particularly preferred are the C_(1–C) ₆ linear or branched alkylesters. Specific examples are ethylbenzoate, n-butylbenzoate, p-methoxyethylbenzoate, p-ethoxy ethylbenzoate, isobutylbenzoate, ethylp-toluate, diethyl phthalate, di-n-propyl phthalate, di-n-butylphthalate, di-n-pentyl phthalate, di-i-pentyl phthalate,bis(2-ethylhexyl) phthalate, ethyl-isobutyl phthalate, ethyl-n-butylphthalate, di-n-hexyl phthalate, di-isobutylphthalate. The electrondonor compound is normally present in amounts such as to give aTi_((tot))/ED molar ratio of higher than 1, preferably higher than 2.5and more preferably higher than 4. As explained above, the catalystcomponent (A) comprises, in addition to the above electron donors, Ticompounds in different valence state, Mg and halogen. In particular, thecatalyst component comprises in addition to the above electron donors,titanium compounds having at least a Ti-halogen bond and a Mg dihalide.The magnesium halide is preferably MgCl₂ in active form which is widelyknown from the patent literature as a support for Ziegler-Nattacatalysts. Patents U.S. Pat. No. 4,298,718 and U.S. Pat. No. 4,495,338were the first to describe the use of these compounds in Ziegler-Nattacatalysis. It is known from these patents that the magnesium dihalidesin active form used as support or co-support in components of catalystsfor the polymerization of olefins are characterized by X-ray spectra inwhich the most intense diffraction line that appears in the ASTM-cardreference of the spectrum of the non-active halide is diminished inintensity and broadened. In the X-ray spectra of preferred magnesiumdihalides in active form said most intense line is diminished inintensity and replaced by a halo whose maximum intensity is displacedtowards lower angles relative to that of the most intense line.

The preferred titanium compounds used in the catalyst component of thepresent invention are the halides of Ti, in particular among those inwhich the Ti has valence 4, TiCl₄, and among those in which the Ti hasvalence lower than 4 TiCl₃; furthermore, can also be usedTi-haloalcoholates of formula Ti(OR^(I))_(n−y)X_(Y), where n is thevalence of titanium, y is a number between 1 and n, X is halogen,preferably chlorine, and R^(I) is a C1–C15 hydrocarbon group. Asexplained, the amount and kind of Ti compounds must be such that to havethe ratio Ti^((red))/Tt_((tot)) ranging from about 0.05 to about 1.

The preparation of the solid catalyst component (A) can be carried outaccording to several methods. One of the preferred methods comprises thepreparation of a solid comprising titanium compounds with a valencehigher than 3, Mg compounds and electron donor and then the reaction ofthis solid with a compound capable to reduce at least partially thetitanium compounds in order to form Ti compounds with different valencestates meeting the above ratio requirement. The preparation of a solidcatalyst component comprising titanium compounds with a valence higherthan 3, Mg compounds and electron donor can be carried out according toseveral methods. According to one of these methods, the magnesiumdichloride in an anhydrous state and the electron donor compound aremilled together under conditions such that activation of the magnesiumdichloride occurs. The so obtained product can be treated one or moretimes with an excess of TiCl₄ at a temperature between 80 and 135° C.This treatment is followed by washings with hydrocarbon solvents untilchloride ions disappeared. According to a further method, the productobtained by co-milling the magnesium chloride in an anhydrous state, thetitanium compound and the electron donor compound is treated withhalogenated hydrocarbons such as 1,2-dichloroethane, chlorobenzene,dichloromethane etc. The treatment is carried out for a time between 1and 4 hours and at temperature of from 40° C. to the boiling point ofthe halogenated hydrocarbon. The product obtained is then generallywashed with inert hydrocarbon solvents such as hexane.

According to another method, magnesium dichloride is preactivatedaccording to well known methods and then treated with an excess of TiCl₄at a temperature of about 80 to 135° C. which contains, in solution, theelectron donor compound. The treatment with TiCl₄ is repeated and thesolid is washed with hydrocarbon solvents in order to eliminate anynon-reacted TiCl₄.

According to a preferred method, the solid catalyst component can beprepared by reacting a titanium compound of formulaTi(OR^(I))_(4−y)X_(y), preferably TiCl₄, where y is a number between 1and 4, and X and R^(I) have the meaning previously explained, with amagnesium chloride deriving from an adduct of formula MgCl₂●pR^(II)OH,where p is a number between 0,1 and 6, preferably from 2 to 3.5, andR^(II) is a hydrocarbon radical having 1–18 carbon atoms. The adduct canbe suitably prepared in spherical form by mixing alcohol and magnesiumchloride in the presence of an inert hydrocarbon immiscible with theadduct, operating under stirring conditions at the melting temperatureof the adduct (100–130° C.). Then, the emulsion is quickly quenched,thereby causing the solidification of the adduct in form of sphericalparticles. Examples of spherical adducts prepared according to thisprocedure are described in U.S. Pat. No. 4,399,054 and U.S. Pat. No.4,469,648. The so obtained adduct can be directly reacted with the Ticompound or it can be previously subjected to thermal controlleddealcoholation (80–130° C.) so as to obtain an adduct in which thenumber of moles of alcohol is generally lower than 3 preferably between0,1 and 2,5. The reaction with the Ti compound can be carried out bysuspending the adduct (dealcoholated or as such) in cold TiCl₄(generally 0° C.); the mixture is heated up to 80–130° C. and kept atthis temperature for 0,5–2 hours. The treatment with TiCl₄ can becarried out one or more times. The electron donor compound can be addedduring the treatment with TiCl₄. The treatment with the electron donorcompound can be repeated one or more times.

The preparation of catalyst components in spherical form is describedfor example in European Patent Applications EP-A-395083, EP-A-553805,EP-A-553806, EPA-601525 and WO98/44009. The solid catalyst componentsobtained according to the above method show a surface area (by B.E.T.method) generally between 20 and 500 m²/g and preferably between 50 and400 m²/g, and a total porosity (by B.E.T. method) higher than 0,2 cm³/gpreferably between 0,2 and 0,6 cm³/g. The porosity (Hg method) due topores with radius up to 10.000 Å generally ranges from 0.3 to 1.5 cm³/g,preferably from 0.45 to 1 cm³/g. In any of these preparation methods thedesired electron donor compound can be added as such or, in analternative way, it can be obtained in situ by using an appropriateprecursor capable to be transformed in the desired electron donorcompound by means, for example, of known chemical reactions such asesterification, transesterification etc.

As explained above the so obtained solid is then reacted with a compoundhaving a reducing ability with respect to the titanium atoms with avalence of 4. This compound may be any compound that, on the basis ofits red/ox characteristics, one skilled in the art can expect to beeffective. Examples of such compounds are organometallic compounds suchas organoaluminum compounds or polyhydrosiloxanes. Particularlypreferred are organoaluminum compounds of formula AIR^(III) _(a)X_(3−a)where a is from 1 to 3, R^(III) is a C1–C15 hydrocarbon group, X ishalogen. Preferably, a is 3 and R is a C1–C10 alkyl group.

The contact of the reducing compound with the solid disclosed above mustoccur under conditions such that the reducing compound is effective ingiving the ratio Ti^((red))/Ti_((tot)) ranging from about 0.05 toabout 1. This means that conditions such reaction time, temperature andconcentration of reactants must be suitably selected to get the requiredproperties. For example, working with very diluted systems or too shortreaction times or in the presence of compounds that lower the reducingabilities of the reducing compound may not be suitable for obtaining therequired product. The contact preferably occurs in the presence of aliquid medium that can be selected from liquid organic substances.Preferably it is selected from liquid aliphatic or aromatichydrocarbons, optionally halogenated, such as pentane, hexane,dichloromethane, benzene, toluene etc. It has been found convenient towork with these systems with a concentration of solid ranging from 1 to300 g/l, while the reducing compound concentrations can be lower orhigher depending on the reducing capabilities. Generally, reaction timesof from 1 minute to several hours are used. Such times however, cansometimes be shorter or longer depending also on the reducingcapabilities and on the concentration of the reactants.

The contact may be carried out one or more times. At the end of thetreatment with the reducing agent the solid catalyst component obtainedcan be washed with inert solvents and then dried.

The solid catalyst component (B) is preferably selected from thetrialkyl aluminum compounds such as for example triethylaluminum,triisobutylaluminum, tri-n-butylaluminum, tri-n-hexylaluminum,tri-n-octylaluminum. It is also possible to use mixtures oftrialkylaluminum compounds with alkylaluminum halides, alkylaluminumhydrides or alkylaluminum sesquichlorides such as A1Et₂Cl and Al₂Et₃Cl₃.

Optionally the catalyst system used in the process of the invention canalso comprise one or more electron-donor compounds (external donor) ascomponent (C).

The external donor (C) can be of the same type or it can be differentfrom the electron donor compound present in the solid catalyst component(A). Suitable external electron-donor compounds include siliconcompounds, ethers, esters such as ethyl 4-ethoxybenzoate, amines,heterocyclic compounds and particularly 2,2,6,6-tetramethyl piperidine,and ketones. One particular class of preferred external donor compoundsis that of silicon compounds of formula R_(a) ⁵R_(b) ⁶Si(OR⁷)_(c), wherea and b are integer from 0 to 2, c is an integer from 1 to 3 and the sum(a+b+c) is 4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicalswith 1–18 carbon atoms optionally containing heteroatoms. Particularlypreferred are the silicon compounds in which a is 1, b is 1, c is 2, atleast one of R⁵ and R⁶ is selected from branched alkyl, cycloalkyl oraryl groups with 3–10 carbon atoms optionally containing heteroatoms andR⁷ is a C₁–C₁₀ alkyl group, in particular methyl. Examples of suchpreferred silicon compounds are methylcyclohexyldimethoxysilane,diphenyldimethoxysilane, methyl-t-butyldimethoxysilane,dicyclopentyldimethoxysilane,2-ethylpiperidinyl-2-t-butyldimethoxysilane,1,1,1,trifluoropropyl-metil-dimethoxysilane and1,1,1,trifluoropropyl-2-ethylpiperidinyl-dimethoxysilane. Moreover, arealso preferred the silicon compounds in which a is 0, c is 3, R⁶ is abranched alkyl or cycloalkyl group, optionally containing heteroatoms,and R⁷ is methyl. Examples of such preferred silicon compounds arecyclohexyltrimethoxysilane, t-butyltrimethoxysilane andthexyltrimethoxysilane. The electron donor compound (C) is used in suchan amount to give a molar ratio between the organoaluminum compound andsaid electron donor compound (c) of from 0.1 to 500, preferably from 1to 300 and more preferably from 3 to 100. As previously indicated, thesaid process is suitable for preparing a broad range of ethylenepolymers. In particular, linear low density polyethylenes (HDPE, havinga density lower than 0.940 g/cm³) and very-low-density andultra-low-density polyethylenes (VLDPE and ULDPE, having a density lowerthan 0.920 g/cm³, to 0.880 g/cm³) consisting of copolymers of ethylenewith one or more alpha-olefins having from 3 to 12 carbon atoms, havinga mole content of units derived from ethylene of higher than 80% can beprepared. Preferably, the alpha-olefins are selected from propylene,butene-1,4-methyl-1-pentene, hexene-1, octene-1. As mentioned above, thepolymerization of ethylene in mixture with butene, hexene-1 or octene-1is especially preferred. However, the said process is also suitable forthe preparation of, for example, high density ethylene polymers (HDPE,having a density higher than 0.940 g/cm³), comprising ethylenehomopolymers and copolymers of ethylene with alpha-olefins having 3–12carbon atoms; elastomeric copolymers of ethylene and propylene andelastomeric terpolymers of ethylene and propylene with smallerproportions of a diene having a content by weight of units derived fromethylene of between about 30 and 70%. The LLDPE prepared with theprocess of the invention has so improved mechanical properties that theuse of a polymer with a lower density is no longer required and, as aconsequence, also the presence of high content of soluble fraction isavoided. As demonstrated by the working examples reported below, filmmeeting the requested mechanical properties have been produced by usingethylene copolymers obtained with process of the invention and havinghigher density and lower soluble fraction content with respect to thepolymers of the prior art. The working examples also show that the LLDPEpolymers prepared with the process of the invention and having the samedensity values and soluble fraction content as those of the prior art,have improved mechanical properties. It has in particular been foundthat the copolymers of ethylene with one or more alpha-olefins havingfrom 3 to 12 carbon atoms, having a mole content of units derived fromethylene of higher than 80% produced with the process of the inventionare characterized by a ratio between the intrinsic viscosity of thesoluble fraction and the intrinsic viscosity of the whole polymer higherthan 0.8. The polymerization process of the invention can be carried outaccording to known techniques either in liquid or gas phase using, forexample, the known technique of the fluidized bed or under conditionswherein the polymer is mechanically stirred. The catalyst can be used assuch in the polymerization process by introducing it directly into thereactor. However, it constitutes a preferential embodiment theprepolymerization of the catalyst with an olefin. In particular, it isespecially preferred pre-polymerizing ethylene, or propylene or mixturesthereof with one or more α-olefins, said mixtures containing up to 20%by mole of α-olefin, forming amounts of polymer from about 0.1 g pergram of solid component up to about 1000 g per gram of solid catalystcomponent. The pre-polymerization step can be carried out attemperatures from 0 to 80° C. preferably from 5 to 50° C. in liquid orgas-phase. The pre-polymerization step can be performed in-line as apart of a continues polymerization process or separately in a batchprocess. The batch pre-polymerization of the catalyst of the inventionwith ethylene in order to produce an amount of polymer ranging from 0.5to 20 g per gram of catalyst component is particularly preferred. Themain polymerization process of the invention can be carried outaccording to known techniques either in liquid or gas phase using forexample the known technique of the fluidized bed or under conditionswherein the polymer is mechanically stirred. Preferably the process iscarried out in the gas phase. Examples of gas-phase processes wherein itis possible to use the spherical components of the invention aredescribed in WO92/21706, U.S. Pat. No. 5,733,987 and WO93/03078. In thisprocesses a pre-contacting step of the catalyst components, apre-polymerization step and a gas phase polymerization step in one ormore reactors in a series of fluidized or mechanically stirred bed arecomprised.

Therefore, in the case that the polymerization takes place in gas-phase,the process of the invention is suitably carried out according to thefollowing steps:

-   -   (a) contact of the catalyst components in the absence of        polymerizable olefin or optionally in the presence of said        olefin in amounts not greater than 20 g per gram of the solid        component (A);    -   (b) pre-polymerization of ethylene or mixtures thereof with one        or more α-olefins, said mixtures containing up to 20% by mole of        α-olefin, forming amounts of polymer from about 0.1 g per gram        of solid component (A) up to about 1000 g per gram;    -   (c) gas-phase polymerization of one or more olefins CH₂═CHR, in        which R is hydrogen or a hydrocarbon radical having 1–10 carbon        atoms, in one or more fluidized or mechanically stirred bed        reactors using the pre-polymer-catalyst system coming from (b).

As mentioned above, the pre-polymerization step can be carried outseparately in batch. In this case, the pre-polymerized catalyst ispre-contacted according to step (a) with the aluminum alkyl and thendirectly sent to the gas-phase polymerization step (c).

The Molecular Weight of the polymer is normally controlled usinghydrogen or other agents capable to regulate the Molecular Weight. Ifneeded the polymerization process of the invention can be performed intwo or more reactors working under different conditions and optionallyby recycling, at least partially, the polymer which is formed in thesecond reactor to the first reactor. As an example the two or morereactors can work with different concentrations of molecular weightregulator or at different polymerization temperatures or both. Thefollowing examples are given in order to further describe the presentinvention in a non-limiting manner.

Characterization

The properties are determined according to the following methods:

-   Melt Index: measured at 190° C. according to ASTM D-1238 condition    “E” (load of 2.16 Kg) and “F” (load of 21.6 Kg);-   Comonomer content-   1-Butene was determined via Infrared Spectrometry.-   Effective density: ASTM-D 1505-   Intrinsic viscosity [η]: measured in tetraline at 135° C.    Determination of Ti^((red))

0.5 g of the sample in powder form, are dissolved in 100 ml of HCl 2.7Min the presence of solid CO₂. The so obtained solution is then subjectto a volumetric titration with a solution of FeNH₄(SO₄)₂.12H₂O 0.1N, inthe presence of solid CO₂, using as indicator of the equivalence pointNH₄SCN (25% water solution). The stoichiometric calculations based onthe volume of the titration agent consumed give the weight amount ofTi³⁺ in the sample.

Determination of Ti_((tot)).

1 g of the sample in powder form is poured in a 500 ml-glass flask,containing 100 ml of H₂SO₄ 1.8M. The system is left to stay for about 1hour, while stirring to increase the dissolution of the sample. Afterthat, the whole is poured in a known volume flask and brought to exactvolume with water. 100 ml of the solution obtained above are poured in aglass flask containing 50 g of metallic Zn and 50 ml of HCl in order toconvert all the Ti atoms at an oxidation state lower than 4. After about1 hour the solution was filtered in a glass flask containing solid CO₂.It was obtained a clear solution which was titrated according to thesame procedure as that illustrated for the determination of theTi^((red)).

EXAMPLES Example 1

Preparation of the Spherical Support (MgCl₂/EtOH Adduct)

The adduct of magnesium chloride and alcohol was prepared according tothe method described in Example 2 of U.S. Pat. No. 4,399,054, butoperating at 2000 rpm instead of 10,000 rpm.

The adduct containing approximately 3 mol of alcohol had an average sizeof approximately 60 μm, with a dispersion range of approximately 30–90μm.

Preparation of the Solid Component

The spherical support, prepared according to the general method, wassubjected to thermal treatment, under nitrogen flow, within thetemperature range of 50–150° C., until spherical particles having aresidual alcohol content of about 35 wt. % (1.1 mol of alcohol per moleof MgCl₂) were obtained. 16 g of this support were charged, understirring at 0° C., to a 750 cm³ reactor containing 320 cm³ of pure TiCl₄and 3.1 cm³ of diisobutylphtalate, were slowly added and the temperaturewas raised to 100° C. in 90 minutes and kept constant for 120 minutes.Stirring was discontinued, settling was allowed to occur and the liquidphase was removed at the temperature of 80° C. Further 320 cm³ offreshly TiCl₄ were added and the temperature was raised to 120° C. andkept constant for 60 minutes. After 10 minutes settling the liquid phasewas removed at the temperature of 100° C. The residue was washed withanhydrous heptane (300 cm³ at 70° C. then 3 times (250 cm³ each time)with anhydrous hexane at 60° C. and further 4 at ambient temperature.The component in spherical form was vacuum dried at 50° C.

The characteristics of the solid (a) were the following:

Ti_((tot))  2.3 wt. % Ti^((red)) not present Mg 19.8 wt. %diisobutylphtalate  4.2 wt. %

The so obtained solid (a) is then introduced in a 5 l glass reactorcontaining a volume of hexane such as to have a concentration of 55 g/l.An amount of triethylaluminum such as to give a weight ratioTEAL/catalyst of 0.25. The temperature of the system was brought to 30°C. and kept under stirring at this temperature for 30 minutes. Afterthat the liquid phase was siphoned off and the same treatment with TEALrepeated a second time. After having washed the solid with hexane anddried it under vacuum the analysis showed the following composition:

Ti_((tot))  2.1 wt. % Ti^((red))  1.54 Mg 21.1 wt. % diisobutylphtalate 1.6 wt. %Polymerization

The polymerization process was carried out in a plant workingcontinuously and equipped with a reactor in which the catalystcomponents are mixed to form the catalyst, two loop reactors receivingthe catalyst formed in the previous step and fed with liquid propyleneand propane, and one fluidized bed reactor, receiving the pre-polymerformed in the loop reactors. The solid catalyst component prepared asdescribed above, a solution of triethyl aluminium (TEAL) in n-exane andmethyl-cycloexyl-dimethoxysilane as electron-donor in such an amountthat the TEAL/silane weight ratio is 4.5 and the TEAL/Catalyst weightratio is 5.0, were fed into the pre-contact reactor which was kept atthe temperature of 10° C. In the same reactor propane was also fed asinert medium. The residence time was about 9 minutes. The productdischarged from the reactor was then fed into the first looppre-polymerization reactor kept at 20° C. The residence time in the loopreactor was about 32 minutes. After that time the pre-polymer wastransferred to the second loop reactor kept at 50° C. and furtherpolymerization went on for an average residence time of 80 minutes.After that time the product was discharged from the loop reactor andtransferred to the fluidized bed reactor which was maintained at atemperature of 75° C. with a reaction pressure of 24 bar. The averageresidence time of the polymer forming inside the reactor was about 244minutes. The components fed to the fluidized bed reactor were thefollowing:

-   -   ethylene and 1-butene as polymerization monomers;    -   hydrogen as molecular weight regulator;    -   propane as inert medium.

The detailed list of working conditions for each step of the process isreported below:

Pre-contact Step

-   -   temperature (° C.)=10    -   residence time (min.)=9    -   Catalyst feeding (g/h) 10    -   Teal feeding (g/h)=50    -   Donor feeding (g/h)=11    -   propane feeding (Kg/h)=5        First Pre-polymerization Step    -   temperature (° C.)=20    -   residence time (min.)=32    -   propane feeding (Kg/h)=50    -   propylene feeding (Kg/h)=3.5        Second Pre-polymerization Step    -   temperature (° C)=50    -   residence time (min.)=80    -   propane feeding (Kg/h)=20        Gas Phase Reactor    -   temperature (° C.)=75    -   pressure (bar)=24    -   residence time (min)=244    -   ethylene (% mol)=20.9    -   butene-1 (% mol)=9    -   hydrogen (% mol)=6.8    -   propane (% mol)=63.2

The polymer obtained was continuously discharged into the steaming anddrying section of the plant and subsequently characterized. The polymerobtained showed a density of 0.916, a comonomer content of 8% wt andxylene soluble fraction of 14.1% having an intrinsic viscosity of 1.59.The intrinsic viscosity of the whole polymer was 1.78.

With the ethylene copolymer prepared according to the polymerizationprocess disclosed above it was produced a blown film having a thicknessof 25 μm with the properties reported in Table 1.

Example 2

Polymerization

The polymerization of ethylene and butene-1 was carried out with thesame catalyst and apparatus used in example 1 under the followingconditions:

The solid catalyst component prepared as described above, a solution oftriethyl aluminum (TEAL) in n-exane and methyl-cycloexyl dimethoxysilaneas electron-donor in such an amount that the TEAL/silane weight ratio is4.2 and the TEAL/Catalyst weight ratio is 5.2 were fed into thepre-contact reactor which was kept at the temperature of 10° C. In thesame reactor propane was also fed as inert medium. The residence timewas about 9 minutes.

The product discharged from the reactor was then fed into the first looppre-polymerization reactor kept at 20° C. The residence time in the loopreactor was about 32 minutes. After that time the pre-polymer wastransferred to the second loop reactor kept at 50° C. and furtherpolymerization went on for an average residence time of 80 minutes.After that time the product was discharged from the loop reactor andtransferred to the fluidized bed reactor which was maintained at atemperature of 75° C. with a reaction pressure of 24 bar. The averageresidence time of the polymer forming inside the reactor was about 250minutes. The components fed to the fluidized bed reactor were thefollowing:

-   -   ethylene and 1-butene as polymerization monomers;    -   hydrogen as molecular weight regulator;    -   propane as inert medium.

The detailed list of working conditions for each step of the process isreported below:

Pre-contact Step

-   -   temperature (° C.)=10    -   residence time (min.)=9    -   Catalyst feeding (g/h)=10    -   Teal feeding (g/h)=52    -   Donor feeding (g/h)=12.5    -   propane feeding (Kg/h)=5        First Pre-polymerization Step    -   temperature (° C.)=20    -   residence time (min.)=32    -   propane feeding (Kg/h)=50    -   propylene feeding (Kg/h)=3.5        Second Pre-polymerization Step    -   temperature (° C.)=50    -   residence time (min.)=80    -   propane feeding (Kg/h)=20        Gas Phase Reactor    -   temperature (° C.)=75    -   pressure (bar)=24    -   residence time (min)=250    -   ethylene (% mol)=20.5    -   butene-1 (% mol)=7.7    -   hydrogen (% mol)=6.8    -   propane (% mol)=65

The polymer obtained was continuously discharged into the steaming anddrying section of the plant and subsequently characterized.

The polymer obtained showed a density of 0.919, a comonomer content of7.1% wt and xylene soluble fraction of 11.6% having an intrinsicviscosity of 1.5. The intrinsic viscosity of the whole polymer was 1.78.With the ethylene copolymer prepared according to the polymerizationprocess disclosed above it was produced a blown film having a thicknessof 25 μm with the properties reported in Table 1.

Comparison Example 1

A polymerization of ethylene was carried out using the same apparatusand conditions described in example 1 with the only difference that as asolid catalyst component was used the solid (a) of example 1 in whichthe ratio Ti^((red))/Ti_((tot)) is 0. The polymer obtained showed adensity of 0.917, a comonomer content of 8% wt and xylene solublefraction of 14.5% having an intrinsic viscosity of 1.4. The intrinsicviscosity of the whole polymer was 1.82.

With the ethylene copolymer prepared according to the polymerizationprocess disclosed above it was produced a blown film having a thicknessof 25 μm with the properties reported in Table 1.

TABLE 1 EX. 1 EX. 2 COMP. EX. 1 Haze 40 43 35 Gloss 14 13 15 Dart-test117  110  95 Elmendorf TD/MD 435/230 420/180 425/180

1. A process comprising polymerizing ethylene, optionally in mixtureswith olefins CH₂═CHR in which R is a hydrocarbon radical with 1–12carbon atoms, carried out in the presence of a catalyst systemcomprising (A) a solid catalyst component which comprises Mg, halogen,an electron donor selected from ethers, esters, or amines, and Ti atomsin an oxidation state such that the weight percentage ratio betweenTi^((red))/Ti_((tot)) ranges from about 0.05 to about 1 whereinTi^((red)) is the weight percentage on the solid catalyst component ofthe Ti atoms having a valence less than 4 and Ti_((tot)) is the weightpercentage on the solid catalyst component of all the Ti atoms, and (B)an Al-alkyl compound; wherein a molar ratio of Ti_((tot))/electron donoris higher than
 1. 2. A process according to claim 1 in which in thesolid catalyst component (A) the ratio Ti^((red))/Ti_((tot)) ranges from0.1 to 0.9.
 3. A processs according to claim 2 in which the ratioTi^((red))/Ti_((tot)) ranges from 0.2 to 0.85.
 4. A process according toclaim 1 in which in the solid catalyst component (A) the electron donorcompound is selected from ethers and esters of organic mono ordicarboxylic acids.
 5. A process according to claim 4 in which theelectron donor compound is selected from alkyl esters of phthalic acids.6. A process according to claim 4 in which the electron donor compoundis selected from 1,3-diethers of the general formula (I):

wherein R^(I), R^(II), R^(III), R^(IV), R^(V) and R^(VI) equal ordifferent to each other, are hydrogen or hydrocarbon radicals havingfrom 1 to 18 carbon atoms, and R^(VII) and R^(VIII), equal or differentfrom each other, have the same meaning of R^(I)–R^(VI) except that theycannot be hydrogen; one or more of the R^(I)–R^(VIII) groups can belinked to form a cycle.
 7. The process according to claim 1 in which thesolid catalyst component (A) comprises an electron donor, titaniumcompounds having at least a Ti-halogen bond and a Mg dihalide.
 8. Theprocess according to claim 7 in which the titanium compounds are thechlorides of Ti.
 9. The process according to claim 1 in which thecatalyst component (B) is a trialkyl aluminum compound.
 10. The processaccording to claim 9 in which the trialkylaluminum compound is selectedfrom triethylaluminum, triisobutylaluminum, tri-n-butylaluminum,tri-n-hexylaluminum and tri-n-octylaluminum.
 11. The process accordingto claim 1 in which the olefins CH₂═CHR, are selected from propylene,butene-1, 4-methyl-1-pentene, hexene-1, and octene-1.
 12. The processaccording to claim 1 in which the catalyst system further comprises oneor more external electron-donor compounds (C).
 13. The process accordingto claim 12 in which the electron donor compound (C) is a siliconcompound of formula R_(a) ⁵R_(b) ⁶Si(OR⁷)_(c), where a and b areintegers from 0 to 2, c is an integer from 1 to 3 and the sum (a+b+c) is4; R⁵, R⁶, and R⁷, are alkyl, cycloalkyl or aryl radicals with 1–18carbon atoms optionally containing heteroatoms.